1. Field of the Invention
The present invention relates to thermal neutron detectors, and more specifically, it relates to improvements in the detectors as well as improved fabrication techniques.
2. Description of Related Art
Solid-state thermal neutron detectors are required for a variety of applications, particularly, nonproliferation of special nuclear material (SNM). The currently used technology involves 3He tubes, which have a variety of shortcomings when utilized in the field as thermal neutron detectors, including the need for high-voltage operation, sensitivity to microphonics, and large size. Moreover, the limit of the world's supply of 3He presents yet another set of critical issues related to strategic and tactical implications, which are of practical importance.
A variety of monolithic solid-state thermal neutron detectors have been proposed. A commonly used geometry involves the use of a planar semiconductor detector over which a neutron reactive film has been deposited. Upon a surface of the semiconductor detector is attached a coating that releases ionizing radiation reaction products upon interaction with a neutron. The ionizing radiation reaction products can then enter into the semiconductor material of the detector, thereby creating a charge cloud of electrons and holes, which can be sensed to indicate the occurrence of a neutron interaction within the neutron sensitive film. The charges are swept through such configured detectors via methods known by those of ordinary skill in the art and registered as an electrical signal.
Present technology for radiation detection suffers from flexibility and scalability issues. Since neutrons have no charge and do not interact significantly with most materials, special neutron converters in solid form have been used to react with neutrons and generate charged particles that can be easily detected by semiconductor devices to generate electrical signals. As an example, the neutron sensitive materials may include Boron and various compounds thereof, such as, 10Boron; or a compound containing 10Boron, such as natural Boron, natural Boron Carbide, 10Boron Carbide, or 10Boron Nitride. Other classes of neutron sensitive materials include, but are not limited to, Lithium (e.g. pure 6Lithium; or a compound containing 6Lithium such as 6Lithium Fluoride), 155Gadolinium, or 157Gadolinium. Charge collecting materials may include semiconductors (e.g., Si, Ge, etc.) and alloys thereof (e.g., GaAs, InP, etc.), as well as organic semiconductors.
The prior art has been limited to various thin-film, monolithic structures for the detection of thermal neutrons. As an example, one class of solid-state neutron detector is in the form of a planar structure. This device is comprised of a single thin-film layer of the neutron sensitive material, grown onto a semiconductor substrate and, is hence, limited to low detection efficiencies. Beyond single-layer structures, various 2D and 3D designs of detector geometries to collect the generated electron-hole pairs are also being pursued. Thin-film techniques to deposit the neutron sensitive material onto or in the charge collecting material have included e-beam evaporation and chemical vapor deposition.
The prior art is also limited to structures in which the charge-collecting platform and the neutron-sensitive material are well suited for typical fabrication techniques that can support both sets of materials, possibly simultaneously. Hence, the prior art involves the fabrication of, and is limited to, monolithic structures. That is, the monolithic structures must be amenable to existing deposition techniques and materials that are compatible with each other.
Moreover, the choice of working materials using the prior art is constrained to be compatible within a given deposition system so that it can concomitantly employ the desired neutron-sensitive compounds as well as the solid-state semiconductor. As an example, certain neutron-sensitive materials may not be amenable with existing-thin-film deposition systems (vapor, plasma, chemical, epitaxial, etc.), by virtue of differential vapor pressures, deposition temperatures, toxicity, corrosion, radioactivity, contamination, as well as safety issues.
Examples of prior art sensors include fiber optic scintillometers combined with various neutron-sensitive materials, including fibers coated thin layers of equivalent “doped paint,” as well as fiber matrices immersed in doped paraffin or liquids that surround the fibers. In all these prior-art embodiments, the neutron-sensitive material is utilized as a dopant in a liquid or solid, thereby limiting the density of the desired material.
Finally, the prior art involves material compatibility issues, both during device fabrication, and, furthermore, during the in-field usage of the sensor over its expected lifetime. Thin-film material constraints can limit the classes of materials and more importantly the quality of the material, owing to differences and effects in the pressure and temperature during the process. For many vacuum system processes (sputting, electron beam evaporation and CVD) the material will be coated in a line of sight configuration which can be difficult to coat high aspect ratio structures due to shadowing. For a low pressure vacuum CVD with optimized temperature the coating can be conformal and completely fill a high aspect ratio structure but can suffer from undue stress in the material composite which can lead to material delamination and material cracking.